The shortage of high-quality standing lumber has driven researchers and wood products manufacturers to look for alternative low-quality resources for value-added applications. To achieve these goals, suitable technologies are needed to improve specific wood quality attributes (e.g., dimensional stability, durability, mechanical properties, and hardness) in order to meet end-use requirements. Low-quality resources can be modified through different treatments to acquire the attributes necessary to meet specific requirements. Lignocellulosic composite materials have been used in many structural applications as substitutes for traditional lumber. These lignocellulosic composites are made from lignocellulosic materials comprised of wood strands, wood fibers or wood chips, and the wood adhesives (less than 10% based oven dry cellulosic material), and are formed at elevated temperatures and pressures.
Wood adhesives are key components for wood composites. Sellers (2001) reported that North American consumed more than 1.78×108 tons of wood adhesives (solid basis) in 1998, in which urea-formaldehyde (UF) and melamine-formaldehyde (MF) adhesives account for around 60%, and phenol-formaldehyde (PF) adhesive accounts for over 30%. Because of formaldehyde release during the usage of wood composites with UF or MF or melamine-urea-formaldehyde (MUF) adhesives, these adhesives are facing challenges from current and near future regulations. Because of the thermal resistance and weather resistance compared to other wood adhesives such as UF resin, MF resin and isocyanate adhesives, PF resins are commonly used for weather proof particleboard, OSB, MDF and/or plywood for use under exterior weather conditions. Apart from that, PF resin usually has very low formaldehyde emission in service.
PF resin is produced from petroleum products. Petroleum reserves are naturally limited. Thus the wood composites industry would benefit greatly from the development of green lignin-based phenolic resin.
The wood-decomposing fungi can be divided into two groups according to their modes of action on wood materials: brown-rot and white-rot fungi. Brown-rot fungi can degrade wood polysaccharides and leave behind a brown, partially modified (oxidized) lignin residue. Some brown-rot fungi can also produce laccase in liquid culture, but the laccases produced by brown-rot fungi have a low redox potential that allows direct oxidation only of phenolic lignin units, which often comprises less than 10% of the total polymer. White-rot fungi can degrade both polysaccharides and lignin selectively or simultaneously and leave a cellulose-enriched white material. They often invade the lumens of wood cells and cause progressive lignin degradation between fibres.
Hüttermann et al (1989) reviewed the enzymatic modification of lignin for technical use to produce a homogeneous, pure lignin preparation of reasonably high molecular weight with high reactivity provided by reactive functional groups. Ligninase, laccase, and poly-blue-oxidase three enzymes that change the lignin structure through different mechanisms. Ligninase is the main lignolytic system in white-rot fungi, and can catalyze the oxidation of veratryl alcohol. Laccase acts on phenolics via a non-specific oxidation. Presence of laccase showed polymerization of ligninase both in vivo and in vitro. The low molecular weight substances from enzymatically degraded lignin are rarely re-polymerized. Poly-blue-oxidase oxidizes the lignin model compound poly-blue.
Lignin is a recalcitrant molecule that does not lend itself to adhesive manufacture without modification. One way of doing this is using wood-decomposing fungi (Jin et al 1990).
Li (2005) made a presentation about developing green wood adhesives from renewable natural resources. He mentioned that brown-rot fungi preferentially degrade carbohydrates, but do not substantially depolymerise lignin. The brown-rot fungi demethylate lignin and oxidize lignin side chains to some extent. He used natural brown-rot fungi-decayed Douglas-fir wood from the forest dissolved in a dilute sodium hydroxide solution and the soluble decayed wood was reduced with sodium hydroxide, and mixed with polyethyleneimine used for wood adhesives.
Jin et al (1990) extracted extensively brown-rotted Douglas-fir wood by refluxing with 0.1N sodium hydroxide, acidified the solution with 0.1N hydrochloric acid to precipitate lignin (BRL) (pH 3-4), then centrifuged, washed and freeze-dried it. The phenol-formaldehyde resin with 35% wt BRL substitution of phenol was formulated.